26P/Grigg–Skjellerup

Comet Grigg–Skjellerup (formally designated 26P/Grigg–Skjellerup) is a periodic comet. It was visited by the Giotto probe in July 1992.^[6] The spacecraft came as close as 200 km, but could not take pictures because some instruments were damaged from its encounter with Halley's Comet.^[7] The comet last came to perihelion (closest approach to the Sun) on 25 December 2023,^[2][1][4] but was 1.8 AU from Earth and only 31 degrees from the Sun.^[1]

26P/Grigg–Skjellerup

Discovery
Discovered by	John Grigg and John Francis Skjellerup
Discovery date	1902 and 1922
Designations
Alternative designations	1808 C1; 1808 III; 1902 O1; 1902 II; 1902c; 1922 K1; 1922 I; 1922b; 1927 F1; 1927 V; 1927e; 1932 II; 1932d; 1937 III; 1937e; 1942 V; 1942d; 1947 II; 1947a; 1952 IV; 1952b; 1957 I; 1956i; 1961 IX; 1961g; 1967 I; 1966f; 1972 II; 1972b; 1977 VI; 1977b; 1982 IV; 1982a; 1987 X; 1986m; 1992 XVIII
Orbital characteristics
Epoch	2023-02-25 (JD 2460000.5)
Aphelion	4.947 AU
Perihelion	1.0840 AU
Semi-major axis	3.015 AU
Eccentricity	0.6405
Orbital period	5.23 yr
Max. orbital speed	36.6 km/s @ perihelion[1]
Inclination	22.43°
Last perihelion	October 1, 2018[2] July 6, 2013[3] March 23, 2008
Next perihelion	2023-Dec-25[2][1][4]
Earth MOID	0.1 AU (15 million km)[5]

History of observation

The comet was discovered in 1902 by John Grigg of New Zealand, and rediscovered in its next appearance in 1922 by John Francis Skjellerup, an Australian then living and working for about two decades in South Africa where he was a founder member of the Astronomical Society of Southern Africa. In 1987, it was belatedly discovered by Ľubor Kresák that the comet had been observed in 1808 as well, by Jean-Louis Pons. Pons observed the comet on 6 and 9 February, which was insufficient to calculate an approximate orbit.^[8]

In 1972 the comet was discovered to produce a meteor shower, the Pi Puppids, and its current orbit makes them peak around April 23, for observers in the southern hemisphere, best seen when the comet is near perihelion.

During the comet's 1982 approach it was detected using radar by the Arecibo Observatory.^[9]

Having its recent perihelion so close to Earth's orbit made it an easy target to reach for the Giotto spacecraft in 1992, whose primary mission was to Comet Halley. Giotto had a closest approach to Grigg–Skjellerup of 200 km, much closer than its approach to Comet Halley, but was unable to obtain images as its camera was destroyed during the Halley rendezvous in 1986.

The 2002 return (expected perihelion around October 8, 2002) was very unfavorable and no observations were reported.

Properties

The comet has often suffered the gravitational influence of Jupiter, which has altered its orbit considerably. For instance, its perihelion distance has changed from 0.77 AU in 1725 to 0.89 AU in 1922 to 0.99 AU in 1977 and to 1.12 AU in 1999.

The comet nucleus is estimated to be 2.6 kilometers in diameter.^[5]

The comet is a type locality for the mineral brownleeite.^[10][11]

Popular culture

• In Neal Stephenson's science fiction novel Seveneves, 26P/Grigg–Skjellerup serves as a potential source of water and rocket propellant for the "Cloud Ark" survivors, and is frequently referred to by the nickname "Greg's Skeleton" by way of homophonic transformation.^{[citation needed]}